Repeating Aftershocks of the 16th April 2016 Mw 7.8 Pedernales (Ecuador) Earthquake Underline the Interplay Between Afterslip and Seismicity
Résumé
Repeating earthquakes are earthquakes that repeatedly break a single, time-invariant fault patch. They are generally associated with aseismic slip, which is thought to load asperities, leading to repeated rupture. Repeating earthquakes are therefore useful tools to study aseismic slip and fault mechanics, with possible applications to earthquake triggering, loading rates and predictability. In this study, we analyze one year of aftershocks following the 16th April 2016 Mw 7.8 Pedernales earthquake in Ecuador to find repeating families, using data recorded by permanent and temporary seismological networks. We focus on a small area north of the mainshock containing about 900 catalogued events, where seismicity during both the inter-seismic and post-seismic periods has been previously linked to aseismic slip. We calculate waveform cross-correlation coefficients (CC) on all available catalogue events, which we use to sort 195 events into 84 preliminary families of 2 to 18 events, using a minimum CC of 0.9. These events were then stacked and used to perform template-matching on the continuous data. In total, 387 earthquakes were classified into families, including 12 from the one-year period before the mainshock. We later relocated these earthquakes using a double-difference method, which confirmed that most of them did have overlapping sources. Repeating earthquakes seem to concentrate largely around the area of largest afterslip release. We also find an increase in the recurrence time of repeating events with time after the mainshock over the year of the postseismic period. Overall, our results suggest that most aftershocks are driven by afterslip release. The presence of new repeating families beyond the first month after the mainshock could be a sign of afterslip migration downdip from the early postseismic areas near the trench. Meanwhile, the increase in repeating earthquakes' recurrence times with time highlights a possible timeframe for the afterslip's deceleration.